1. Field of the Invention
The present invention relates to a device for controlling the heating of a catalyst for purifying an exhaust gas.
2. Description of the Related Art
A catalyst for purifying the exhaust gas is normally arranged in the exhaust passage of the engine, and when the engine is started, the temperature of the catalyst is gradually increased due to the heating thereof by the exhaust gas. This exhaust gas purifying operation by the catalyst, however, cannot be properly carried out until the catalyst is activated, i.e., until the temperature of the catalyst reaches an activation temperature, and accordingly, the exhaust gas cannot be purified by the catalyst until the temperature of the catalyst reaches the activation temperature thereof after the engine is started.
Therefore, in a known engine, an electric heater is provided for heating the catalyst and is supplied with electric power for a predetermined time after the engine is started (see Japanese Unexamined Utility Model Publication No. 47-22313). In this engine, when the engine is started, the catalyst is rapidly heated, and accordingly, the temperature of the catalyst rises rapidly to the activation temperature thereof, to thus purify the exhaust gas shortly after the engine is started.
If the catalyst is used for a long time, however, and thus is deteriorated, the activation temperature at which the catalyst starts the proper purifying operation of the exhaust gas becomes gradually higher as the catalyst deteriorates. Nevertheless, as in the above-mentioned engine, where the electric heater is supplied with electric power for a predetermined time after the engine is started, the temperature of the catalyst is increased only to a predetermined temperature, regardless of the degree of deterioration of the catalyst.
Accordingly, in this case, if the time for which the electric heater is supplied with electric power is determined such that the temperature of the catalyst rises to the activation temperature when the catalyst is new, the temperature of the catalyst cannot be raised to the activation temperature before the supply of electric power to the electric heater is stopped, when the catalyst has deteriorated. Accordingly, a problem arises in that, when the catalyst has deteriorated, the exhaust gas cannot be purified for a short time after supply of electric power to the electric heater is stopped.
Conversely, by taking the deterioration of the catalyst into consideration, if the time for which the electric heater is supplied with electric power is determined such that the temperature of the catalyst still rises to the activation temperature even though the catalyst has deteriorated, the temperature of the catalyst becomes considerably higher than the activation temperature before the supply of power to the electric heater is stopped, when the catalyst is new. Accordingly, in this case, a problem arises in that unnecessary energy is used for heating the catalyst, and further, the catalyst may be deteriorated sooner due to the excessive heating operation by the electric heater.